Could killer horse virus spread amongst humans?

Australia is suffering the biggest outbreak of the highly virulent Hendra virus since the disease was identified in 1994.

Now a change in its symptoms in Queensland horses is raising fears that new strains may have emerged – and even that a strain capable of spreading from human-to-human could appear.

“The different clinical presentations, and some very preliminary [DNA] sequencing data, suggest that the Hendra virus may be somewhat different in this outbreak,” says epidemiologist Hume Field of the Australian Biosecurity Cooperative Research Centre for Emerging Infectious Diseases in Brisbane.

“It all suggests that there may be a cluster of Hendra virus strains, all closely related, but differing in their pathogenesis and infectivity,” he says.

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Two people, both veterinary clinic staff, who became infected roughly four weeks ago, remain in hospital. Fifty more people who may have had contact with horses carrying the virus will undergo a second set of tests over the next few days.

Delayed diagnosis

At least seven horses have become infected with the virus, and five have died.

Thirty-six horses will be tested for a second time tomorrow, and over 90 horses have been tested since the start of the outbreak, says chief veterinary officer Ron Glanville of Biosecurity Queensland in Brisbane.

The classic symptom of Hendra virus infection in a horse is severely laboured breathing, sometimes with a frothy nasal discharge and a swollen muzzle.

The animals often deteriorate rapidly, and die with in days. But in the current outbreak the horses have mainly suffered from neurological symptoms such as paralysis and loss of balance. “One owner said the horse appeared drunk,” says Glanville.

The change in the clinical symptoms has delayed diagnosis and enabled the disease to spread more widely in the current outbreak.

Fruit-bat reservoir

The natural reservoir for the Hendra virus is the fruit bat, which carries the virus without becoming sick, but it remains unclear how the virus “spills over” to horses.

“We are trying to understand why spill over occurs in some years, and not others,” says Field. “That way we can manage the risk better, and prevent Hendra virus becoming an established disease of livestock or humans.”

Hendra virus was first identified in 1994, when 14 horses and one person, a horse trainer, died in the Brisbane suburb of Hendra. A second person was infected, but recovered.

In an outbreak that occurred a few weeks before the index case, and was later recognised as Hendra, two horses died, as did their owner – some 13 months after being infected with the deadly virus.

But every subsequent outbreak until now has been contained, with the virus infecting one horse each time. In one of those outbreaks one person was also infected, but survived.

Symptoms in humans have included a flu-like illness, which can progress to pneumonia; headache, high fever, and drowsiness, which can progress to convulsions or coma.